Increased public awareness regarding the hazards associated with exposure of human skin to ultraviolet (UV) radiation has contributed to the increasing popularity of UV-absorbing or scattering compounds (sunscreens), both independently and as a component of cosmetic products. However, the effectiveness of sunscreen products is directly linked to the efficiency of photo absorption/scattering in the UV range of electromagnetic radiation, in particular, UV-B (280-320 nm) and UV-A (320-400 nm).
Amongst the most widely used combinations of UV-absorbing compounds marketed today is 4-tert-butyl-4-methoxy-dibenzoylmethane (BMDBM) and octyl methoxycinnamate (OMC). Dibenzoylmethane derivatives, including BMDBM, are commonly selected as sunscreen components based on their ability to absorb the full spectrum of UV-A radiation (320-400 nm). Similarly, OMC is known for effectively absorbing electromagnetic radiation in the UV-B range (280-320 nm). However, neither compound alone or in combination is ideal due to their known photo-instability.
Exposure of a dibenzoylmethane derivative to UV-A radiation causes the excitation of an electron in the dibenzoylmethane derivative molecule from an initially occupied, lower energy orbital to a higher energy, previously unoccupied orbital. See Turro, Modern Molecular Photochemistry, 1991. In the electronically excited state, the dibenzoylmethane derivative is prone to degrade via the number of known pathways producing non-UVA-absorbing species and, therefore, can absorb little or no additional UVA energy. Similarly, exposure of OMC to UV radiation transforms the compound into a less absorbing cis-isomer, and thereby less effective at rendering UV absorption. Furthermore, when a dibenzoylmethane derivative (e.g., BMDBM) and OMC are exposed together to UV radiation, a photo reaction, known as the de Mayo reaction, occurs between the species resulting in the rapid disappearance of both species and, consequently, a loss of photoprotective effect.
To photostabilize an electronically excited UV-absorbing organic molecule (e.g., BMDBM), it must be returned to the ground state before it can undergo a photochemical reaction that is destructive to its UV radiation absorbing capabilities.